Service-oriented computing is a new computing paradigm that utilizes services as fundamental elements for developing applications. Service composition plays a very important role in it. This paper focuses on service composition triggered by service requirement. Here, the processes modeling the requirement should be treated in parallel with describing service and a same ontology should be adopted for allowing the understanding between the requirement and services. An effect-based approach has been proposed based on our previous work on service description. This approach could be promising for tackling the challenge of services composition.